The present invention relates to methods and apparatus for measuring the pressure of a fluid medium capable of use in a wide variety of applications. The invention is particularly useful as a vacuum gauge, pressure gauge, barometer, altimeter, Pitot tube speedometer, and helicopter blade leak detector; and the invention is therefore described below with respect to such applications.
The measurement of fluid medium pressure is utilized in many diverse applications, and many different types of pressure measuring devices have been developed for each particular application. One class of pressure measuring devices, particularly used in vacuum systems, is based on the measurement of changes in thermal conductivity accompanying changes in pressure, and thereby changes in density, of the gas. Thus, in a thermal conductivity gauge, the pressure depends on the heat conduction through the gas from a hot spot of a self-regulated source to the surrounding glass or metal envelope at room temperature. A popular example of a thermal conductivity gauge is the Pirani Gauge, in which the hot spot is produced by driving current through a tungsten filament having a linear temperature coefficient of resistance. However, such gauges require temperatures of over 1,000° C., and generally are of large dimensions, cumbersome and of high cost. Also, if this gauge is accidentally exposed to atmospheric environment, the filament is immediately burnt up due to its high temperature.
Moreover, the accuracy and reproducibility of the measurements in a vacuum system are generally just as important as the production of the vacuum. The known thermal conductivity gauges in general, and specifically the Pirani Gauge, are generally characterized by reproducibility or error which, at best, is about 5%. Moreover, because of the extremely large range of pressures that may be involved in a vacuum system, from atmospheric down to 10−12 bar or less, there is no single vacuum gauge which covers the whole range.
The foregoing drawbacks in thermal conductivity gauges in general, and in the Pirani Gauge in particular, are also applicable in other fluid medium pressure measuring devices, such as in barometers for measuring barometric pressure, altimeters, etc.